Logic lighting control system

ABSTRACT

A lighting control system comprises a photosensor (12), a comparative circuit (16) and a logic switching circuit (26). The photosensor (12) produces an output (VP) which is a function of the light intensity level, and the comparative circuit (16) compares the output (VP) from the photosensor (12) with a number of preset levels (V1 to V6) in order to produce a logic output which controls the switching of the logic switching circuit (26) between four states: daylight (32), dusk (34), night (36) and dawn (38). At dusk, when the light intensity falls below a predetermined value (L2) the light is switched on. If the light intensity level then improves the light will not, however, switch off again until the light intensity rises above another higher predetermined level (L6). Similarly at dawn, the light will switch off when the light intensity rises above a predetermined value (L4) and then only switch on again if the light intensity falls below a different, lower predetermined value (L5). The problem of repeated switching on and off as a result of trasient light intensity variations at dusk and dawn is consequently avoided, without the need to introduce significant delays.

The present invention relates to lighting control systems and inparticular lighting control systems utilising photoelectric means tosense light intensity levels, such as are used to control streetlighting automatically.

With conventional lighting control systems which use photoelectriccontrol, a photoelectric sensor is used which is separate from theswitching circuit.

With conventional street lighting systems using photoelectric control,the lights are normally set to switch on at dusk when the lightintensity falls below a first predetermined value and switch off at dawnwhen the light intensity rises above a second predetermined value.Because the street lights take time to reach full intensity, the firstpredetermined value is normally greater than the second predeterminedvalue.

If at dusk, after the lights have switched on, the light intensity levelrises again above the first predetermined value, the lights will switchoff and similarly at dawn, if after the lights have switched off, thelight intensity level falls again below the second predetermined value,the lights will switch on again. As a consequence, the system issensitive to transient light intensity variations which would cause thelights to repeatedly switch on and off as dusk and dawn approach.

In order to overcome this problem, switching of the system is subject tosignificant time delays, for example of the order of 15 to 30 seconds,so that at dusk the lights will not switch on until the light intensityhas remained at or below the first predetermined level for the delayperiod, and at dawn the lights will not switch off until the lightintensity has remained at or above the second predetermined level forthe delay period.

This system has the drawback that the switching on of the lights isdelayed for a significant period during which light levels maydeteriorate substantially. This then affects the accuracy of thecontrol.

According to one aspect of the present invention a lighting controlsystem comprises a photosensor, comparator means and a logic switchingcircuit, wherein the photosensor produces an output which is a functionof the light intensity level, the comparator means being adapted tocompare the output from the photosensor with a plurality of presetlevels and to produce a logic output which controls the switching of thelogic switching circuit between a plurality of states.

Preferably the lighting control system is in the form of an integratedcircuit.

According to a preferred embodiment of the invention the light fallingupon the photosensor is filtered to provide a response from thephotosensor which is close to the human eye.

According to a further aspect of the present invention the logicswitching circuit has four states; the logic switching circuit being ina first state when the light intensity is above a first predeterminedvalue;

the logic switching circuit going from its first state to a second statewhen the light intensity falls below a second predetermined value;

the logic switching circuit going from its second state to a third statewhen the light intensity falls below a third predetermined value;

the logic switching circuit going from its third state to a fourth statewhen the light intensity rises above a fourth predetermined value;

the logic switching circuit going from its fourth state to its thirdstate when the light intensity falls below a fifth predetermined value;

the logic switching circuit going from its second state to its firststate when the light intensity rises above a sixth predetermined value;

the fifth predetermined value being below the fourth predetermined valueand the sixth predetermined value being above the second predeterminedvalue.

Switching means is preferably provided for switching the lighting systemon when the logic switching circuit is in its second and third statesand switching the lighting system off when the logic switching circuitis in its first and fourth states.

With the above system, the first state of the logic switching circuitcorresponds to normal daylight, the second state to dusk, the thirdstate to night and the fourth state to dawn. At dusk, when the lightintensity falls below the second predetermined value the light isswitched on. If the light intensity level improves the light will not,however, switch off again until the light intensity rises above thesixth predetermined level. Similarly at dawn, the light will switch offwhen the light intensity rises above a fourth predetermined value andthen only switch on again if the light intensity falls below the fifthpredetermined value. The problem with transient light intensityvariations is consequently avoided without the need to introducesignificant delays. A short delay may be incorporated into the controlsystem in order to prevent the logic switching circuit changing stateprematurely with transient variations in light intensity. However, asthe sensitivity of the control system to transient variations in lightintensity at dusk and dawn is very much reduced, much shorter timedelays may be used, thereby avoiding the problems of the conventionalcontrol systems.

According to a preferred embodiment of the invention, the sixthpredetermined value may be equal to the first predetermined value andthe fifth predetermined value is intermediate of the third and fourthpredetermined values. The second predetermined value is also preferablyhigher than the fourth predetermined value.

Further embodiments of the invention are defined in the dependentclaims.

An embodiment of the invention is now described, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of a lighting control system in accordance with thepresent invention; and

FIG. 2 is a logic diagram of the lighting control system illustrated inFIG. 1.

FIG. 1 illustrates an integrated circuit 10 for controlling theswitching of a street lighting system in accordance with the lightintensity level sensed by a silicon photosensor 12. The photosensor 12senses the amount of daylight and produces a voltage which is a functionof the light intensity level.

The voltage from the silicon photosensor 12 is amplified by amplifier 14and is fed to a comparative circuit where it is compared to voltagescorresponding to five predetermined light intensity levels;

voltage V1 corresponding to a first predetermined light intensity levelL1 which represents normal daylight;

voltage V2 corresponding to a second predetermined light intensity levelL2 at which it is desired to switch the lighting system on at dusk;

voltage V3 corresponding to a third predetermined light intensity levelL3 which represents night;

voltage V4 corresponding to a fourth predetermined light intensity levelL4 at which it is desired to switch the lighting system off at dawn; and

voltage V5 corresponding to a fifth predetermined light intensity levelL5 which is intermediate of the third and fourth levels.

Three further inputs 18, 20 and 22 are provided to the comparator 16;input 18 being connected to adjustable means by which the voltage V2 maybe set to a required value; input 20 being connected to adjustable meansby which the voltage V4 may be set to a required value; and input 22being connected to adjustable means by which the voltages V1, V3 and V5may be set to required values. The voltages V1, V3 and V5 areinterrelated so that they may be increased or decreased using the input22.

The comparator 16 compares the voltage VP from the photoelectricdetector 12, with the voltages V1 and V5 and produces a coded logicoutput, when the light intensity produces a voltage VP which correspondsto one of the set voltages V1 to V5.

The logic output from comparator 16 is passed via a low pass filter 24to a logic switching circuit 26. The low pass filter 24 introduces atime delay of five seconds on switching on and ten seconds on switchingoff.

The logic switching circuit 26 has four states corresponding to normaldaylight, dusk, night and dawn. When the light intensity measured by thephotosensor 12 rises to a level L1 corresponding to voltage V1, andremains at the level or above for more than 10 seconds, the logic signalfrom the comparator 16 will switch the logic switching circuit 26 to thedaylight state 32. If the light intensity measured by the photosensor12, falls to a level L2 corresponding to voltage V2 or below, andremains at that level or below for more than five seconds, the logicswitching circuit 26 will be switched to the dusk state 34. If the lightintensity level continues to deteriorate and falls to a level L3corresponding to voltage V3 or below and remains there for more thanfive seconds, the logic switching circuit 26 will go to the night state36. If however while the logic switching circuit 26 is in the duskstate, the light improves to an intensity level L1 corresponding tovoltage V1 or greater and remains at that level for ten seconds, thelogic switching circuit 26 will be switched back to the daylight state32. Once the logic switching circuit 26 is in the night state, if thelight improves to an intensity level L4 corresponding to voltage V4 andremains at that level for ten seconds, the logic switching circuit 26will go to the dawn state 38. Further improvements in the light to anintensity level L1 corresponding to voltage V1 which remains at thatlevel for 10 seconds, will then switch the logic switching circuit 26back to the daytime state 32. If however while the logic switchingcircuit 26 is in the dawn state, the light deteriorates to an intensitylevel L5 corresponding to voltage V5 or below and remains at that levelfor a period in excess of five seconds, the logic switching circuit 26will be switched back to the night state 36.

The logic switching circuit 26 produces a pair of outputs which willcontrol the switching on and switching off of the lighting system 30,through a driving circuit 28, so that when the logic switching circuit26 is in the daytime and dawn states, the lighting system will beswitched off and when the logic switching circuit 26 is in the dusk andnight states, the lighting system 30 will be switched on. The drivingcircuit 28 comprises a relay and triac. The relay is controlled by thefirst output from the logic circuit 26, the second output from the logicswitching circuit 26 switching the triac, so that the power may be fedthrough the relay or triac to the lighting system 30. The logicswitching circuit 26 controls the first and second power outputs so thatwhen switching on the lighting system 30 the second output will operatebefore the first output and when switching off the lighting system 30,the first output will be disabled before the second output.

As illustrated by the logic diagram in FIG. 2, when the light levels areabove a first predetermined value for example 200 lux, the logicswitching circuit 26 will be in the daylight state and the lightingsystem 30 will be switched off. When the light level falls to a secondpredetermined intensity level, for example 70 lux, and remains at orbelow the level for five seconds, the logic switching circuit 26 goes tothe dusk state and the lighting system 30 is switched on. If the lightcontinues to deteriorate and reaches a third predetermined value, forexample 2 lux, and remains at that level or below for five seconds, thelogic switching circuit 26 goes to the night state. If however, beforereaching the third predetermined level, the light intensity improves andreaches the first predetermined level (200 lux) and remains at thatlevel for ten seconds, the logic switching circuit 26 switches back tothe daylight state and the lighting system is switched off. When theswitching state logic circuit is in the night state, when the lightintensity reached a fourth predetermined level, for example, 35 lux andremains at that level for ten seconds, the logic switching circuit 26goes to the dawn state and the lighting system 30 is switched off.Continued improvement in the light intensity level back to the firstpredetermined level (200 lux) will then switch the logic switchingcircuit 26 back to the daylight state. If however before reaching thefirst predetermined level, the light intensity deteriorates to below afifth predetermined intensity level, for example 17 lux, and remains atthat level for five seconds, the logic switching circuit 26 returns toits night mode and the lighting system 30 is switched back on.

Various modifications may be made without departing from the invention.For example, while in the embodiment described above, voltages V1, V3and V5 are interlinked and controlled together, each of the voltages mayalternatively be controlled separately or in any other appropriategrouping. While in the above embodiment, when the logic switchingcircuit 26 is in the dusk state 34, the lights will be switched back offif the light intensity improves to the first predetermined value L1,switching back on of the light may alternatively be controlled at asixth predetermined value L6 which is intermediate of the first andsecond predetermined values L1 and L2. In this case a sixth voltage V6,corresponding to the sixth predetermined light intensity level L6, maybe set to the required value by adjustable means connected to the inputs18, 20, 22 which are provided to the comparator 16.

The low pass filter 24 of the circuit described above is preferablyadjustable so that the delay may be varied between zero and 30 secondson switching on and zero and 60 seconds on switching off. While in theabove embodiment the time delay on switching off is twice that onswitching on, any desired relationship between these times may be used.

While it may be desirable in certain circumstances to use longer timedelays, generally time delays significantly shorter than those used inlighting systems hitherto will be used thus overcoming the problem ofcontrol systems used hitherto.

I claim:
 1. A lighting control system comprising a photosensor,comparator means and a logic switching circuit, the photosensorproducing an output which is a function of the light intensity level,the comparator means being adapted to compare the output from thephotosensor with a plurality of preset levels and to produce a logicoutput which controls the switching of the logic switching circuitbetween a plurality of states, wherein the logic switching circuit hasfour states;the logic switching circuit being in a first state when thelight intensity is above a first predetermined value; the logicswitching circuit going from its first state to a second state when thelight intensity falls below a second predetermined value; the logicswitching circuit going from its second state to a third state when thelight intensity falls below a third predetermined value; the logicswitching circuit going from its third state to a fourth state when thelight intensity rises above a fourth predetermined value; the logicswitching circuit going from its fourth state to its third state whenthe light intensity falls below a fifth predetermined value; the logicswitching circuit going from its second state to its first state whenthe light intensity rises above a sixth predetermined value; the fifthpredetermined value being below the fourth predetermined value and thesixth predetermined value being above the second predetermined value. 2.A method of controlling lighting using a photosensor and a logicswitching circuit, the photosensor producing an output which is afunction of the light intensity level, the output being compared with aplurality of preset levels to produce a logic output which controls theswitching of the logic switching circuit between a plurality of states,wherein the logic switching circuit has four states;the logic switchingcircuit being in a first state when the light intensity is above a firstpredetermined value; the logic switching circuit going from its firststate to a second state when the light intensity falls below a secondpredetermined value; the logic switching circuit going from its secondstate to a third state when the light intensity falls below a thirdpredetermined value; the logic switching circuit going from its thirdstate to a fourth state when the light intensity rises above a fourthpredetermined value; the logic switching circuit going from its fourthstate to its third state when the light intensity falls below a fifthpredetermined value; the logic switching circuit going from its secondstate to its first state when the light intensity rises above a sixthpredetermined value; the fifth predetermined value being below thefourth predetermined value and the sixth predetermined value being abovethe second predetermined value.
 3. A lighting control system accordingto claim 1, wherein the photosensor, comparator means and logicswitching circuit form part of an integrated circuit.
 4. A lightingcontrol system according to claim 1, further comprising a lightfiltering means coupled with the photosensor, such that light fallingupon the photosensor is filtered to provide a response to lightintensity from the photosensor which is close to the response of thehuman eye.
 5. A lighting control system according to claim 1, furthercomprising a lighting switching means adapted to switch the lighting onwhen the logic switching circuit is in its second and third states andto switch the lighting off when the logic switching circuit is in itsfirst and fourth states.
 6. A lighting control system according to claim1, further comprising a time delay means which introduces apredetermined time delay between the logic output from the comparatormeans indicative that the light intensity has fallen below or risenabove a predetermined value and the switching of the logic switchingcircuit between states.
 7. A lighting control system according to claim6, wherein the predetermined time delay is less than 60 seconds,preferably less than 30 seconds and most preferably less than 15seconds.
 8. A lighting control system according to claim 6, wherein thepredetermined time delay is greater when switching to the first orfourth states than when switching to the second or third states,preferably by a factor of two.
 9. A lighting control system according toclaim 6, wherein the predetermined time delay when switching to thefirst or fourth states is approximately 10 seconds and when switching tothe second or third states is approximately 5 seconds.
 10. A lightingcontrol system according to claim 1, wherein the sixth predeterminedvalue is equal to the first predetermined value.
 11. A lighting controlsystem according to claim 1, wherein the fifth predetermined value isintermediate of the third and fourth predetermined values.
 12. Alighting control system according to claim 1, wherein the secondpredetermined value is higher than the fourth predetermined value.
 13. Alighting control system according to claim 1, further comprisingadjustment means whereby one or more predetermined output levels,corresponding to one or more predetermined values of light intensity,may be set to a required value.
 14. A lighting control system accordingto claim 6, further comprising time delay adjustment means whereby thepredetermined time delay may be set to a required value.